Steam engines have been in existence for over 150 years. For many years, the best performing type of steam engine was the reciprocating piston driven steam engine. Only within the last fifty years has the turbine steam engine overshadowed the reciprocating piston driven steam engine in industrial use.
The drawbacks of the reciprocating piston steam engine include mass reversal of the piston as it reaches the maximum stroke. Also, the piston is unable to transmit full power during particular positions of the crankshaft. The piston also requires superheated steam to prevent water residue damage in the piston cylinder during operation. Super-heated steam is not efficient to produce. High friction seals are also required.
The drawbacks of the turbine steam engine include the need for high steam pressure to create blade movement and, like the piston steam engine, superheated steam is required to minimize erosion of the turbine blades. Also, large gear reduction assemblies are required to convert high RPM-low torque to low RPM-high torque.
Turbine steam engines are used in many industrial and military applications. For example, power companies use turbine steam engines in their power plants to generate electricity. Turbine steam engines also are used in nuclear-powered navel ships. In many applications using turbine steam engines, nuclear and coal-fired heat can be used to produce the steam.
Rotary expander steam engines are well known in the art. Early rotary expander steam engines are described in, for example, U.S. Pat. No. 137,065 to Fisher, and U.S. Pat. No. 525,121 to Shepard. In rotary expander steam engines, there is no reciprocating piston. Instead, each piston is coupled to a rotating wheel that moves continuously in a single direction. The piston slides or is engaged with the inner walls and surfaces of an steam tight enclosure. Steam is piped into the enclosure to pressurize inner chambers and to drive the piston forward, and steam is exhausted at other locations.
Rotary expander steam engines have certain technical advantages over conventional piston and turbine steam engines. For example, rotary expander steam engines do not require superheated steam during operation. Instead, they can use wet steam because the pistons move in a single direction and do not experience compression. In a wet steam environment, it is practical to use additives such as oil and antifreeze because wet steam will not break down the additives, unlike superheated steam.
Although many rotary expander steam engines have been conceived, existing expander steam engines are deficient due to complexities of the engine. For example, one of the biggest problems in conventional rotary expander steam engines is leakage, which can be difficult to overcome due to the complexity of the piston parts and housings. In particular, existing designs suffer from leakage of steam around the pistons, load shafts and inner engine housings. Furthermore, because steam pressure acts in all directions, it can be difficult to control pressure to directionally drive pistons rotationally forward.